Macroscopic quantum coherence in non-equilibrium and driven quantum systems

My research studies systems in which quantum mechanical effects can beobserved in macroscopic systems. With thousands or millions ofparticles at relatively high temperatures (such as room temperature)most effects of quantum mechanics are washed out. Hence, most thingsin day to day life are entirely classical. There are however largesystems in which quantum mechanical effects can be seen at mediumtemperatures, these can arise when a phase transition to a quantumcondensate occurs. Examples of such condensates includesuperconductivity (where there is flow of current without electricalresistance) and superfluidity of liquid Helium (where there is fluidflow without mechanical resistance). These are striking examples ofhow ``more is different'': In systems of many interacting particles,collective phenomena can arise, where no such effect would be apparentwith only a few particles.Superconductivity and superfluid Helium are however somewhatexceptional as quantum condensates: they are the true equilibriumstates of the given material. The last decade has seen an increasingrange of other quantum condensates in systems which are not in perfectequilibrium. These include cold dilute gases of alkali atoms and veryrecently condensates of quasi-particle excitations in semiconductors,microcavity polaritons. Microcavity polaritons are mixtures ofphotons (quantised particles of light) and excitons (quantisedpolarisation of the semiconductor); this mixing is achieved usingmirrors to build a cavity that confines light, and placing a quantumwell that confines excitons between these mirrors. Microcavitypolaritons can form quantum condensates at much higher temperaturesthan the cold atomic gases, but are further from equilibrium due tothe finite lifetime of the polaritons. While the equilibriumcondensate, and the highly non-equilibrium laser have been extensivelystudied, exploration of systems between these two limits has onlybegun recently. This will be a major area of my research.In addition to allowing the investigation of coherence out ofequilibrium, the new quantum condensates have other differences fromprevious condensates; these include the effect of confining acondensate to two dimensions and the consideration of particles whoseinternal structure is relevant. Combining the effects of reduction totwo dimensions, internal structure, and nonequilibrium behaviour, thedescription of coherence in these systems can differ significantlyfrom previous examples of condensates. An understanding of how andwhen these differences inhibit the formation of quantum condensates isimportant both in terms of producing quantum condensates under easilyattainable conditions (i.e. room temperature), and in extending thevariety of properties that these condensates may have.Another area in which non-equilibrium many-body quantum mechanicalproblems arise is when parameters of the system (e.g. appliedelectric and magnetic fields, applied laser beams) are deliberatelyvaried in time. In many cases it is sufficient to describe suchsystems classically, since systems with many particles often wash outquantum mechanical effects; however there are examples, such asvarying parameters near the transition to a quantum condensate whenquantum effects should be seen. Building on recent examples ofstrongly coupled light-matter systems in which semiclassicaltreatments are inadequate, I will study whether there are cases wherethere can be dramatic signatures of quantum mechanics in such drivensystems.In summary, my work aims to study the conditions under whichmacroscopic physical systems show quantum behaviour, to consider whatuses this behaviour may have, and to understand what is required toextend the range of conditions where such behaviour can be seen.